The present invention relates generally to a semiconductor device and in particular concerns a supporting electrode brazed to a semiconductor substrate of the semiconductor device, which electrode is made of a composite material containing carbon fibers in a matrix of copper.
In many semiconductor devices, the semiconductor substrate is brazed onto electrodes which serve also as the support for the substrate. In order that thermal stress produced during fabrication or operation of the semiconductor device be abated or suppressed and that heat generated in the semiconductor substrate be satisfactorily dissipated, it is required that the electrode should have following properties:
(1) Thermal expansion coefficient of the electrode should approximate to that of the semiconductor substrate as closely as possible. PA1 (2) The electrode should exhibit a high thermal conductivity.
Tungsten and molybdenum have heretofore been used as preferred electrode materials for satisfying the requirements mentioned above. However, in view of the fact that the coefficient of thermal expansion (hereinafter referred to as the thermal expansion coefficient) of silicon of which the semiconductor substrate is made is 3.5.times.10.sup.-6 /.degree.C. in contrast to those of tungsten and molybdenum which are 4.5.times.10.sup.-6 /.degree.C. and 5.5.times.10.sup.-6 /.degree.C., respectively, it can not always be said that tungsten and molybdenum are very satisfactory. Further, thermal conductivities of these elements are poor when compared with copper.
As an electrode material which is to replace tungsten and molybdenum, a composite material in which carbon fibers are embedded in copper matrix has been proposed (refer to U.S. Pat. No. 3,969,754, for example). This composite material enjoys both a low thermal expansibility attributable to the carbon fibers and a high thermal conductivity ascribable to the presence of copper. Besides, by adjusting the content of the admixed carbon fibers, it is possible to make the thermal expansion coefficient of the composite material coincide with that of the substrate material. However, when the content of the carbon fibers is increased, the thermal expansion coefficient of the composite electrode material is decreased and at the same time the thermal conductivity is also decreased because of inherently poor thermal conductivity of the carbon fibers per se. Under the circumstance, there is a demand for realization of an electrode of such a composite material in which thermal conductivity is improved by reducing the content of the admixed carbon fibers while satisfying the requirement of the low thermal expansibility. When an electrode of the composite material which meets the above demand is employed in a semiconductor device, the semiconductor substrate will then be substantially insusceptible to thermal stress, while heat generated in the substrate can be dissipated in a satisfactory manner. Thus, with such an electrode, a semiconductor device would be obtained which can enjoy a high stability with respect to electrical and mechanical characteristics and which can assure satisfactory operation even in the state where a large current is handled.